The present invention relates to a detector and a method for detecting the presence/absence and/or nature of a security mark on an article.
In the field of product or document security it is known to place a machine readable mark on an article or its packaging, which mark can be read by a detector to identify the article and/or to verify the authenticity or otherwise of the article. Such a mark may typically be printed on the article, and may be invisible to the naked eye.
For example, there exist particularly sophisticated, complex inks which reliably emit radiation with certain characteristics under exposure to radiation in a certain frequency range. Such complex inks, which are by their nature difficult for counterfeiters to manufacture include inks known as taggant inks.
A taggant-ink marking on an article will, when exposed to appropriate radiation, exhibit behaviour of a certain signature or characteristic peculiar to that ink. For example, when the ink sample is irradiated by radiation at an “excitation” frequency, the ink sample will emit radiation, at an emission frequency, and will continue to do so after exposure to the excitation radiation has ceased. The emitted radiation after excitation has ceased decays in a known, repeatable manner which is unique to the particular taggant.
One example of taggant material, as used in such complex inks, comprises a base material of lattice structure which includes one or more rare-earth metal dopants. By varying the level of dopant, or the position of dopant molecules within the lattice it is possible to produce a range of taggant materials which exhibit different, but predictable repeatable characteristics when excited by a radiation source.
Typically a detector is used to provide the excitation radiation and then to detect emitted radiation to determine the presence/absence, and in certain cases the signature or characteristic, of ink on the article or its packaging.
The detector must therefore incorporate some form of radiation source, the frequency of which must be known, precise and reliably repeatable, and a detection mechanism which is able to detect the presence of emitted radiation of the appropriate frequency.
Previously considered techniques have many problems. Firstly, since the taggant materials most commonly used emit frequency-shifted radiation in the visible part of the spectrum, ambient light levels can make measurement of the emitted radiation difficult, as can coloured backgrounds or substrates. Furthermore, this type of process produces a low power output—i.e. the emitted radiation is very weak.